Electrical distribution systems, for example for distributing an electrical power supply through a building or industrial facility, are often subjected to harmonic currents generated by non-linear loads such as electronic equipment (including computers, adjustable speed drives (ASD), uninterruptable power supplies (UPS), power rectifiers, etc.) and equipment that uses different kinds of arc processes (including arc discharge lighting systems). These harmonic-generating loads generate various levels of conventional harmonics (5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th etc.) and, for single phase line-to-neutral non-linear loads, also zero phase sequence or "triplen" harmonics (3rd, 9th etc.) in the power distribution system, the harmonic spectrum depending upon the nature of the harmonic-generating load.
For example, FIG. 1 illustrates a typical current consumption waveform of a computer load at 60 Hz fundamental frequency, the accompanying table illustrating the distribution of harmonic currents present in the power distribution system as a percentage of the fundamental current. FIG. 2 illustrates a typical current consumption waveform of an ASD at 60 Hz, the accompanying graph illustrating the distribution of harmonic currents present in the power distribution system.
These harmonic currents create many problems in the power distribution system, including increased voltage total harmonic distortion level, reduced electromagnetic compatibility of the loads, reduced reliability of the power distribution equipment, increased power losses, reduced power factor, and other problems which are well known to those skilled in the art.
Prior art systems for mitigating harmonic currents fall into five basic types:
1. Power factor corrected (PFC) power supplies: In these systems the rectified current is continually adjusted to smooth the current consumption waveform. An example is illustrated in FIG. 3. PFC's are relatively expensive devices and their applications are limited. Also, PFC's cannot be retrofitted for use with existing power supplies, and are not practical for use with large ASD's. PA1 2. Active filters: These devices inject into the conductors between the power distribution system and the load, harmonic currents having a polarity opposite to those generated by the load, thereby neutralizing harmonic currents flowing into the power distribution system. An example is illustrated in FIG. 4. Active filters have many disadvantages, including high cost, poor reliability and poor dynamic characteristics. Active filters also are not practical for use with large ASD's. PA1 3. Resonant L-C filters: L-C filters are commonly used in power systems, tuned to different harmonic frequencies to mitigate specific harmonic currents. An example is illustrated in FIG. 5. These devices present many problems which are well known to those skilled in the art, including high cost, poor effectiveness in low voltage systems and the tendency to cause the system to operate with a leading power factor. Further, because L-C filters are non-directional they are easily overloaded by untreated harmonic currents generated by other harmonic sources connected to the power distribution system (for example in a neighboring facility), resulting in overloading and frequent failures of the filter's capacitor bank. PA1 4. AC chokes: In this harmonic mitigating technique reactors are connected in series between the line and the load. An example is illustrated in FIG. 6a (without a core) and 6b (with a core). This technique is simple, reliable and relatively low cost, however it results in a high voltage drop across the reactors. To reduce the voltage drop one must reduce the choke reactance level, which commensurately reduces the effectiveness of the choke and substantially limits harmonic current mitigation. PA1 5. Phase shifting systems: Different kinds of phase shifters are available which allow the creation of quasi-multiphase systems, reducing certain harmonic levels. Harmonic currents of targeted orders are cancelled or substantially reduced depending upon the selected degree of the phase shift. However, such systems are typically limited in terms of the number of harmonic orders which can be mitigated, and the degree of harmonic mitigation depends upon the extent to which harmonics produced by the various harmonic sources are identical and their phase shift angles.
The voltage can be boosted by connecting a capacitor bank between the load and the choke, as shown in FIG. 7, but this frequently causes the system to operate with a leading power factor (especially in the case of light loading). In this case, since the reactance of the reactor at harmonic frequencies is much higher than the reactance of the reactor at the fundamental frequency, a large portion of the harmonic currents drain to the neutral through the capacitor. The capacitor has a high reactance at the fundamental frequency. However, the voltage drop across the choke remains very high. Thus, large compensating capacitors must be connected between the load and the choke to boost the voltage, which substantially increases the size and cost of the system and causes the system to operate at increased voltage levels during light loading conditions.